Nonstationary signal analysis of magnetic islands in plasmas

Rotating magnetic islands produce fluctuations on a variety of diagnostics in magnetic fusion energy plasmas. The analysis of these fluctuations requi res the calculation of the amplitude, phase, and frequency of the oscillati ons. These three spectral quantities generally evolve in time, necessitatin g nonstationary signal analysis techniques. The Hilbert transform offers an efficient and accurate method of calculating these three quantities from o ne diagnostic signal. This feature allows the Hilbert transform to determin e the success of the active rotation control of magnetic islands, and to ca lculate the profile of the diagnostic measurements in a frame of reference co-rotating with the magnetic island. Comparisons to quadrature and spectro gram techniques demonstrate the accuracy of the Hilbert transform method. ( C) 1999 American Institute of Physics. [S0034-6748(99)04812-1].